1. Field of the Invention
This invention relates to a bipolar separator plate for use in polymer electrolyte membrane (PEM) fuel cells. More particularly, this invention relates to a liquid cooled, bipolar sheet metal separator plate for use in polymer electrolyte membrane fuel cells. Although the concept of this invention may be applied to bipolar separator plates for a variety of fuel cell designs, it is particularly suitable for use in polymer electrolyte membrane fuel cell stacks in which the fuel and oxidant are provided to each of the fuel cell units comprising the fuel cell stack through internal manifolds.
2. Description of Prior Art
There are a number of fuel cell systems currently in existence and/or under development which have been designed and are proposed for use in a variety of applications including power generation, automobiles, and other applications where environmental pollution is to be avoided. These include molten carbonate fuel cells, solid oxide fuel cells, phosphoric acid fuel cells, and polymer electrolyte membrane fuel cells. One issue associated with successful operation of each of these fuel cell types is the control of fuel cell temperature and the removal of products generated by the electrochemical reactions from within the fuel cell.
Polymer electrolyte membrane fuel cells are particularly advantageous because they are capable of providing potentially high energy output while possessing both low weight and low volume. Polymer electrolyte membrane fuel cells are well known in the art. Each such fuel cell comprises a "membrane-electrode-assembly" comprising a thin, proton-conductive, polymer membrane-electrolyte having an anode electrode film formed on one face thereof and a cathode electrode film formed on the opposite face thereof. In general, such membrane-electrolytes are made from ion exchange resins, and typically comprise a perflourinated sulfonic acid polymer such as NAFION.TM. available from E.I. DuPont DeNemours & Co. The anode and cathode films typically comprise finely divided carbon particles, very finely divided catalytic particles supported on the internal and external surfaces of the carbon particles, and proton-conductive material intermingled with the catalytic and carbon particles, or catalytic particles dispersed throughout a polytetrafluoroethylene (PTFE) binder.
The membrane-electrode-assembly for each fuel cell is sandwiched between a pair of electrically conductive elements which serve as current collectors for the anode/cathode and frequently contain an array of grooves in the faces thereof for distributing the fuel cell gaseous reactants over the surfaces of the respective anode and cathode.
Commercially viable fuel cell stacks may contain up to about 600 individual fuel cells (or fuel cell units), each having a planar area up to several square feet. In a fuel cell stack, a plurality of fuel cell units are stacked together in electrical series, separated between the anode electrode of one fuel cell unit and the cathode electrode of an adjacent fuel cell unit by an impermeable, electrically conductive, bipolar separator plate which provides reactant gas distribution on both external faces thereof, which conducts electrical current between the anode of one cell and the cathode of the adjacent cell in the stack, and which, in most cases, includes the internal passages therein which are defined by internal heat exchange faces and through which coolant flows to remove heat from the stack. Such a bipolar separator plate is taught, for example, by U.S. Pat. No. 5,776,624. In such fuel cell stacks, the fuel is introduced between one face of the separator plate and the anode side of the electrolyte and oxidant is introduced between the other face of the separator plate and the cathode side of a second electrolyte.
Cell stacks containing 600 cells can be up to several feet tall, presenting serious problems with respect to maintaining cell integrity during heat-up and operation of the fuel cell stack. Due to thermal gradients between the cell assembly and cell operating conditions, differential thermal expansions, and the necessary strength of materials required for the various components, close tolerances and very difficult engineering problems are presented. In this regard, cell temperature control is highly significant and, if it is not accomplished with a minimum temperature gradient, uniform current density will not be maintainable, and degradation of the cell will occur.
In addition to temperature considerations, fuel cell stack integrity is also a function of the physical dimensions of the stack. The larger the fuel cell stack, the more difficult it becomes to maintain stack integrity and operation. Accordingly, in addition to temperature control, for a given electrical output which is a function of the number of fuel cell units comprising the fuel cell stack, it is desirable that the fuel cell stack dimensions, in particular, the fuel cell stack height be as small as possible for a given electrical output.